Stabilization of electroless plating solutions



United States Patent Oiz'fice 3,515,564 STABILIZATION OF ELECTROLESS PLATING SOLUTIONS Glenn O. Mallory, Jr., Inglewood, and Donald W. Baudrand, Temple City, Calif., assignors, by mesne assignments, to Allied Research Products, Inc., Baltimore, Md., a corporation of Maryland No Drawing. Filed May 27, 1968, Ser. No. 732,091

Int. Cl. C23c 3/02 U.S. Cl. 106-1 Claims ABSTRACT OF THE DISCLOSURE The spontaneous reaction of nickel with contaminants present in an electroless nickel plating bath is significantly arrested by addition thereto of a sulfonic acid stabilizing compound. Isethionic acid and salts thereof, such as sodium isethionate or potassium isethionate, as Well as vinyl sulfonic acid and its salts, have proven to be especially valuable in extending the operating life of the plating solutions.

BACKGROUND OF THE INVENTION This inventon relates to a process for stabilizing electroless plating baths and more particularly concerns the stabilization of a plating solution containing nickel cations and hypophosphite anions.

Electroless deposition is the process by which a metal is reduced from a solution and deposited onto a catalytic surface of an article through oxidation of a reducing agent at that surface. Deposition is accomplished by immersing the article in the electroless plating bath for a time suflicient to obtain the desired thickness. Since electric current is not employed, there is no requirement that contact be made to the surface of the article. For a given substrate material to be plated, the deposition rate can be controlled by the composition and temperature of the plating bath.

The electroless plating of nickel from an aqueous bath containing nickel cations and hypophosphite anions is occasioned by the catalytic reduction of the nickel cations to metallic nickel and the corresponding oxidation of hypophosphite anions to phosphite anions with the evolution of hydrogen gas at the catalytic surface. The reactions take place when the article to be plated is immersed in the plating bath, the exterior surface of the article being coated with nickel.

A significant deficiency in present nickel plating technology consists of the tendency of the plating bath toward decomposition. Organic contaminants present in the bath, such as oils and greases, and metallic contaminants, such as zinc or cadmium, occasion the random chemical reduction of the nickel cations, with the usual result that a black precipitate is formed on the walls and bottom of the bath container. As a consequence, the bath composition typically is of short storage life and both the quality and quantity of the deposited metal are often deficient. Moreover, the plating rate is usually significantly retarded.

A variety of complexing, sequestering or chelating agents, generally referred to as stabilizing agents, have been added to electroless plating compositions to obtain improved plating results. U.S. Pat. No. 2,847,327 discloses a number of long chain aliphatic organic compounds, including fatty acids, amino compounds, sulfates, and the like, which function in a quasi-stabilizing capacity by means of controlling the surface tension of the plating bath. Divalent sulfur compounds, such as thiourea, have also been employed in electroless nickel baths,

3,515,564 Patented June 2, 1970 as disclosed in U.S. Pat. No. 2,762,723. Such materials, as well as the other stabilizing agents in present use, have failed to significantly extend the effective operable life of the baths.

STATEMENT OF THE INVENTION This invention contemplates the stabilization of electroless nickel and/or cobalt plating solutions by the addition thereto of a sulfonic acid compound selected from the group consisting of isethionic acid, vinyl sulfonic acid and their salts.

When used in minor amounts, these stabilizing compounds have been found to react with or otherwise tie up the various organic and/or metallic bath contaminants, thereby avoidng significant random chemical reduction of the nickel cations. Moreover, such materials do not tend to decrease the deposition rate, as is occasioned with the use of many conventional stabilizers. The addition of such materials to nickel plating solutions significantly increases the effective life of such baths.

The salts of isethionic acids are also effective in controlling contamination of plating baths using boron-containing reducing agents, such as dimethyl amine borane. The salts are particularly valuable in systems Where nonmetallic substrates are to be plated by means of sensitizing the substrate with palladium, platinum, gold, or similar metals, prior to immersion in an electroless nickel bath. They are also useful for retarding solution decomposition normally occuring with the use of the above materials.

Although the invention will be specifically described with respect to nickel plating baths, it is to be understood that cobalt baths, as well as plating baths containing mixtures or alloys of such metals, can also be stabilized by means of the described stabilizing compounds. A typical cobalt plating bath contains ingredients substantially identical to those employed in a nickel bath, the source of cobalt cations being supplied by compounds such as cobalt chloride, cobalt sulfate, and similar salts.

The stabilizing compounds of this invention can be employed with any conventional electroless nickel and/or cobalt plating solution. The plating bath typically comprises an aqueous solution containing nickel cations, hypophosphite anions, buffering agents, and stabilizing compounds. Conventional stabilizing compounds, such as ammonia compounds, can also be employed in combination with the compounds of this invention, if desired. The nickel cations are usually derived from nickel salts, such as nickel chloride, nickel sulfate, and the like; the hypophosphite anions from sodium, potassium, lithium, and similar hypophosphites, or combinations thereof. Buffering agents, which typically include an appropriate weak acid or base, are utilized to achieve a desired pH level in the solution. Any other nickel plating bath, such as a solution utilizing an ammonium complexing agent and wherein the nickel is added as nickel ammonium glycolate, can be stabilized in accordance with the method of this invention. Similarly, conventional plating rates and operating procedures are employed.

Although sulfates and sulfonates of fatty acids and fatty alcohols have been utilized as stabilizing materials, as disclosed in the above mentioned patent, there has been no such utilization of isethionic acid. The sulfonate materials referred to in U.S. Pat. No. 2,847,327, noted above, relate to the compounds formed by reaction of sulfonic acids with fatty acids. It has now been found that isethionic acid, and its like-behaving homologues, yield unexpected advantages in electroless nickel plating by means of significantly extending the effective life of the plating bath as well as by providing greatly improved resistance to random reduction of the nickel cations.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Isethionic acid and its salts comprise an especially preferred class of stabilizing compounds for use in the process of this invention. Nickel or cobalt plating solutions containing minor amounts of such materials have proven to be remarkably stable. Specific salts of the acid include: sodium isethionate, potassium isethionate, calcium isethionate, magnesium isethionate, nickel isethionate, cobalt isethionate, lithium isethionate, and the like.

Vinyl sulfonic acid and its salts also constitute a valuable class of stabilizing materials. Among the salts of such acid are included: sodium vinyl sulfonate, potassium vinyl sulfonate, ammonium vinyl sulfonate, calcium vinyl sulfonate, magnesium vinyl sulfonate, nickel vinyl sulfonate, cobalt vinyl sulfonate, lithium vinyl sulfonate, and the like.

The stabilizing compounds of this invention are preferably added to an electroless nickel or cobalt plating solution in amounts within a range of from about 1 to 50 grams per liter of the bath composition. However, amounts as high as 100 grams per liter can be utilized, depending on the concentration of contaminants in the bath. Combinations of salts, such as mixtures of sodium isethionate and sodium vinyl sulfonate, can also be employed in accordance with this invention.

Articles which can be plated in accordance with the process of this invention include metals such as iron, cobalt, nickel, and the like, which are catalytic for the oxidation of all commonly used reducing agents; metals such as copper, silver, gold and the like, which may be indirectly plated after an initial displacement of catalytic material thereon; as well as plastic and other materials treated so as to allow the electroless deposition of nickel thereon.

The article to be nickel plated is cleaned, and/or otherwise treated in accordance with standard practices employed in electroplating processes, and immersed in a suitable volume of aqueous bath containing nickel cations, reducing anions (preferably hypophosphite anions), buffering agents, and the stabilizing compounds. Any other ingredients useful to the desired end product may also be included. The bath is adjusted to an optimum pH value, preferably within a range of from about 7 to 11. The bath is then heated, ordinarily to a temperature below its boiling point at atmospheric pressure. Deposition of metallic nickel on the immersed article proceeds, as indicated by escaping hydrogen, until the required thickness of the nickel coating has been deposited. The coated article is then removed from the bath and rinsed with Water, after which it is ready for use.

In carrying out the process in a continuous plating operation, the stabilizing compound is usually fed periodically or continuously into the plating bath along with other regenerating chemicals, so as to keep the level constant at the desired concentration. That is, the stabilizing compound is added along with the other regenerating ingredients to keep the level of concentration of the stabilizing compounds within the preferred range.

In any or all of the above steps, conventional electroless plating conditions and procedures may be employed.

The following formulations are provided to illustrate typical electroless nickel plating baths for employment in accordance with the process of this invention.

Sodium isethionate Ammonium carbonate is added to the bath to adjust the pH to a value of from 7 to 10.5. Nickel ammonium glycolate is the source of nickel cations and sodium hypophosphite provides the reducing anions.

BATH NO. 2

Gramsfliter Preferred Elements concentration Range Nickel ammonium glycolate 4. 5 2. 0-15 Sodium hypophosphite 25 10-50 Potassium isethionate 12 10 Ing.-5O

The pH of the bath is maintained at a value of about 5.5.

BATH NO. 3

Grams liter Preferred Elements concentration Range Nickel ammonium glycolate 4 3. 5-15 Ammonium carbonate 24 5-50 Potassium citrate. 15 0-60 Dimethylamine boran 1. 25 0. 5-30 Sodium isethionate 10 0-50 The bath has a pH value of from about 7.0 to 10.5; demethylamine borane being used as the source of reducing anions.

The pH of the bath is maintained at a value of about 10.0.

BATH N 0. 5

Grams/liter Preferred Elements concentration Range Nickel chloride 4. 5 3. 5-15 Potassium citrate 12 0-60 Sodium vinyl sultonate 10 0-50 Dimethylamine borane 10 1-30 Potassium citrate is added to the bath to adjust the pH to a value of about 5.5.

BATH N 0. 6

Grams/liter Preferred Elements concentration Range Nickel chloride 4. 5 2. 0-15 Potassium glycolate 22-. 5 7 5-100 Sodium isethionate. 10 0-50 Sodlum hypophosphiteu 30 10-50 The pH of the bath is maintained at a value of about 5 .5.

It is claimed:

1. In a process for stabilizing an aqueous electroless plating bath comprising a source of metal ions selected from the group consisting of nickel ions, cobalt ions and their mixtures and a complexing and a reducing agent therefor, the improvement comprising adding thereto a compound selected from the group consisting of isethionic acid, vinyl sulfonic acid, their salts and mixtures of said salts and said acids in amounts sufiicient to stabilize said bath.

2. The process of claim 1 wherein the stabilizing compound is isethionic acid or a salt thereof.

3. The process of claim 2 wherein the salt is sodium isethi-onate, potassium isethionate, ammonium isethionate,

5 calcium isethionate, magnesium isethionate, nickel isethionate, cobalt isethionate or lithium isethionate.

4. The process of claim 1 wherein the stabilizing compound is vinyl sulfonic acid or a salt thereof.

5. The process of claim 4 wherein the salt is sodium vinyl sulfonate, potassium vinyl sulfonate, ammonium vinyl sulfonate, calcium vinyl sulfonate, magnesium vinyl sulfonate, nickel vinyl sulfonate, cobalt vinyl sulfonate or lithium vinyl sulfonate.

6. The process of claim 1 wherein the stabilizing compound is added to the bath in an amount within a range of from about 1 to 50 grams per liter of the bath composition.

7. The process of claim 1 wherein the nickel ions are derived from nickel ammonium glycolate, nickel sulfate, or nickel chloride.

8. The process of claim 1 wherein the reducing compound is sodium hypophosphite.

9. The process of claim 8 wherein the pH of the bath composition is maintained within a range of 7 to 11.

10. In an aqueous electroless plating bath composition References Cited UNITED STATES PATENTS 9/1950 Brown 20449 2/1966 Zirngiebl et al. l061 JULIUS FROME, Primary Examiner L. B. HAYES, Assistant Examiner US. Cl. X.R,. 117 130, 47 

